Identifying the composition and sequence of various biomolecules, such as human DNA, with accuracy and specificity is of great interest. Mapping and sequencing technology, however, is time consuming and expensive to develop and implement. For example, sequencing the DNA of a single individual for the Human Genome Project required over $3 billion of funding.
It is estimated that each person's DNA varies from one another by approximately 1 base in 1000. Knowledge of such genetic variations among human populations may allow the scientific community to identify genetic trends that are related to various medical predispositions, conditions, or diseases, and may lead to the realization of truly personalized medicine where treatments are customized for a given individual based on that individual's DNA. A reduction in the time and cost of DNA mapping and sequencing is needed to develop such knowledge and to tailor medical diagnostics and treatments based on the genetic makeup of individual patients.
New DNA sequencing technologies produce many short reads (lengths of sequenced DNA) that are then used to assemble the sequence of the entire sample. These “short-read” technologies have sequenced read lengths of from 25 bases to 400 bases. For genomes of modest size or complexity, these short reads are incapable of correctly assembling the sequence of the sample because of the appearance of repeats in the sequence. DNA mapping may be used to guide the assembly. For instance, restriction maps may be used to aid in the assembly of short-read data. More rapid and higher density maps would be useful to enable short-read technologies to assemble data.
Hybridization Assisted Nanopore Sequencing (HANS) is a nanopore-based method for sequencing genomic lengths of DNA and other biomolecules. The method relies on detecting the position of hybridization of probes on specific portions of the biomolecule to be sequenced or characterized.
In this method, two reservoirs of solution are separated by a nanometer-sized hole, or nanopore, that serves as a fluidic constriction of known dimensions. The application of a constant DC voltage between the two reservoirs results in a baseline ionic current that is measured. If an analyte is introduced into a reservoir, it may pass through the fluidic channel and change the observed current, due to a difference in conductivity between the electrolyte solution and analyte. The magnitude of the change in current depends on the volume of electrolyte displaced by the analyte while it is in the fluidic channel. The duration of the current change is related to the amount of time that the analyte takes to pass through the nanopore constriction. The current signals are used in determining the position of the probes on the biomolecule. Measurement of the probe positions allows for accurate reconstruction of the biomolecule sequence using computer algorithms. A need exists for efficient methods and devices capable of rapid and accurate nucleic acid mapping and sequencing for de novo assembly of human genomes. It is desirable to have long read lengths and to use as little nucleic acid template as possible.
Mapping and sequencing may also be achieved using fluidic nano-channel and micro-channel based devices. In these systems, the analyte is caused to transit through a nano- or micro-channel, and its passage is detected by electrodes positioned along the length of the channel. In one embodiment, described in co-pending U.S. patent application Ser. No. 12/789,817, filed May 28, 2010, and the teachings of which are incorporated herein by reference in its entirety, a first pair of electrodes is positioned longitudinally along the channel to provide a reference voltage between them. A second pair of electrodes is positioned across the channel to define a detection volume. As an analyte passes through the detection volume, it causes a detectable change in an electrical property, for example, a change in the reference voltage. Probes that are hybridized to the analyte cause a further change in the electrical property when they enter the detection volume. Thus, it is possible to detect when the analyte is present in the detection volume, as well as the absolute or relative position of hybridized probes as they enter the detection volume.
Despite the foregoing, there remains a need for improved methods and devices for the analysis of biopolymers, including improved assay methods for mapping and sequencing such biopolymers.